The present invention relates to a method and apparatus for testing proximity switches and particularly to such method and apparatus which continuously tests proximity switches by external means.
Proximity switches are used for a variety of checking functions including determination of when and if a particular physical movement has taken place. Sensing devices of this type are used in aircraft control systems to detect door positions, landing gear extension and retraction, wing leading edge slats and flaps position, and air/ground sensing. The proximity switch is ordinarily mounted on a fixed structure and senses the approach of a target, the latter comprising a small piece of metal mounted on a movable component.
Typical maintenance procedure involves physically altering or obstructing the proximity switch/target relationship or manually operating the switch and checking system response. Some aircraft include a microprocessor-controlled tester which can initiate a checking routine if an abnormal target position is indicated. However, some malfunctions may never be detected, and even when an abnormal situation is suspected the checking routine results in nuisance interruptions in the proximity switch output and in the circuit functions controlled thereby.
In a proximity system described in Buck et al U.S. Pat. No. 4,525,699, a proximity circuit is provided with an internally applied test signal superimposed on the regular switch output. This test signal is extracted from the switch output to determine whether the circuit is operating properly. A specialized proximity switch and responsive circuitry is required and the checking function is not applicable to conventional or already existing proximity switches.